In CT systems, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient, and impinges upon a linear array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object. Each detector of the linear array produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
The x-ray source and the linear detector array in a CT system are rotated with a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. A dual beam helical scan may be used to even further reduce the total scan time. Such a scan is made by utilizing a CT system in which two rows of detectors, displayed along the axis of gantry rotation simultaneously collect projection measurements at different axial locations. These two detector rows define two fan beams. In helical scanning, such a system generates interwoven double helixes, as opposed to a single helix from a conventional fan beam helical scan. The interwoven double helixes mapped out by the two fan beams yield projection data from which images in each prescribed slice may be reconstructed with reduced image degradation due to patient translation.
It is desirable, of course, to reconstruct images from the data obtained in a dual beam helical scan in a manner which provides a high quality image with a minimum level of artifacts. It also is desirable to reduce the total time required to reconstruct such an image.